"With this in mind, Charles Dickens’s A Christmas Carol represents two kinds of time travel as it features ghosts who drag a person into the past and into the future. Though Scrooge can’t actually effect the past Marty McFly style, the act of time traveling in his own timestream does alter his present, and in turn, create an alternate reality from the one depicted by the Ghost of Christmas Yet to Come. (One of the reasons why the Doctor Who episode “A Christmas Carol” works so well is because writer Steven Moffat essentially kept this thematic constraint the same.)"